IMHO, it definitely did run it's course naturally.
As others have mentioned you could feel Gene and the regulars getting bored or just tired. Gene had done the show since 1962, albeit a different format, but still. Brett and CNR seemed to be trying to find ways to play fight. Dawson's departure didn't hurt as much as his attitude months before he did leave. Bill Daly and McClean Stevenson were the best semi-regulars after Dawson IMHO of course. The biggest indication for me and reason why I don't like episodes after 78 is the constant editing. Having to edit to time was one thing, but it just seemed to send the message to viewers, rightly or wrongly, that nothing interesting was going on. If they had dubbed the music in post and stopped using the annoying canned laughter, at least it wouldn't have been as obvious.
During the original broadcasts no doubt the time change affected the audience and I guess the ratings didn't help the performers to perform at their best, but even when I see reruns today I just don't find 78 onward enjoyable to watch. The episodes where McLean Stevenson hosted and the "Cuckoo Friend and Ollie" episode were the only ones that stand out for me.
And as much as the updated versions of the classic games are good for today's audience, I just don't find the "in your face" and fake style of television entertaining to watch. Everything is "too polished" now. Some of the funniest moments on MG I enjoyed were when Gene would go off stage and tease the crew or jump off into the audience. I'm sorry, but unless something drastic happens, things like that are never going to happen again, unless they're just shown on YouTube as a behind the scenes or some "Entertainment Tonight" exclusive. Nobody can seem to be satisfied with "live to tape" and just shoot what happens anymore.